The present invention relates to a wear resistant cast iron alloy of high strength and containing spheroidal graphite precipitates for the manufacture of machine parts that are subject to wear, such as piston rings of internal-combustion engines, and specifically small piston rings having a small radial and/or axial wall thickness, and to a method for manufacturing them.
Cast iron alloys for the manufacture of mechanically highly stressed machine parts, such as piston rings of internal-combustion engines, are special alloys, which, in addition to good running and wear characteristics and good elastic behavior, should also have great strength characteristics. In particular, small piston rings having small diameters of, for example, up to 6 mm, have a low absolute strength due to their small axial wall thickness of usually only 1 to 2 mm. Such rings frequently break, even if they are made of cast iron alloys customary for piston rings with larger diameters. Therefore, special cast iron alloys having higher strength values must be used for such small piston rings.
According to C. Englisch, Kolbenringe, in translation "Piston Rings", Volume 1, published by Springer Verlag, Vienna, 1958, pages 204 and 245, such cast iron alloys usually comprise up to 1.0% of chromium, molybdenum, vanadium and copper. Such piston rings are cast so as to ledeburitically white harden, and the desired graphite precipitation and the desired structure are obtained by subsequent heat treatment comprising annealing, quenching and tempering. During the annealing treatment there is a graphitization (graphite precipitation) in which tempered carbon is here obtained in spheroidal form, and as a result of this, the strength of the rings is increased substantially. Evidently, however, as a result of the graphite precipitation, the running and wear characteristics of piston rings made of such alloys are insufficient, so that the bearing faces of the rings must be additionally provided with wear protection layers.
According to DE-AS No. 1,172,049, such cast iron alloys are additionally alloyed with 4.5 to 5.5 percent by weight coopper. In such amounts, copper is present in the form of inclusions which take over the part of more compact graphite precipitates. Piston rings cast from these alloys can therefore also be used with wear protection coatings. But these special alloys are suitable only for large piston rings used in Diesel engines and their strength characteristics are insufficient for small piston rings.
According to DE-OS No. 2,428,822, and corresponding British Pat. No. 1,500,766, there is known a cast iron alloy for piston rings in which the normal spheroidal graphite precipitation is realized with special treatment measures. This alloy can contain as alloying element, in addition to 1.5 to 4.5% silicon, up to 1.7% manganese, up to 2.5% vanadium, up to 2.0% molybdenum, up to 2.0% tungsten, up to 0.8% titanium, up to 1.8% copper, up to 0.5% phosphorus, up to 0.8% nickel, and up to 2.0% niobium. The alloy can be produced by inoculation of the melt with ferrosilicon followed by addition by magnesium in elementary or alloyed form. The phosphorus separates out in phosphorus-containing phases. For refining the individual micro structural constituents, the elements antimony, boron, zirconium and bismuth or also the rare-earth metals may be added, but the sum of these elements must altogether be not more than 0.5 percent by weight. The alloys can be heat treated such as, for example, by annealing above 700.degree. C., followed by quenching and subsequent tempering at temperatures which may be between 200.degree. and 700.degree. C. The alloys have a bainitic to martensic matrix structure. The hardness of these alloys lies between HB 250 and HB 500 kg/cm.sup.2. These alloys have been found to be sufficiently wear resistant, particularly because of their well balanced composition of the alloying elements. However, the relatively compactly encountered spheroidal graphite again adversely influenced the strength in such a way that these alloys were not sufficiently break-resistant to be used for the manufacture of, in particular, small piston rings having low axial heights.
According to DE-OS No. 2,428,821 and corresponding British Pat. No. 1,482,724, there is known a wear-resistant cast iron alloy suitable for construction of machine parts subject to high frictional stresses which contains 1.5 to 4.0% arbon, 1.5 to 6.0% silicon, less than 0.2% sulfur, less than 2.5% phosphorus, 1.0 to 7.0% copper, a total of 0.4 to 3.2% nickel and cobalt, a total of 0.1 to 1.8% tin and antimony, 0.1 to 4.0% molybdenum, 0.1 to 4% tungsten, 0.05 to 2.5% manganese, 0.3 to 2.5% chromium, 0.3 to 4.0% vanadium, 0 to 2.0% titanium, a total of 0.1 to 4.0% niobium and tantalum, 0.1 to 2.0% aluminum, and the remainder iron. The elements boron, bismuth, zirconium, magnesium and/or the rare earth metals can be added. The cast iron alloys contain uncombined carbon as lamellar and primarily nodular precipitates, and a large number of carbides in a very fine crystalline precipitated form. The cast iron alloys are heat treated by annealing above 700.degree. C., such as for one hour at 850.degree. C., quenching to a temperature of below 500.degree. C., such as at room temperature, and subsequently tempering up to a temperature of 700.degree. C., such as at 350.degree. C. for one hour. The cast iron alloys have a bainitic to martensitic basic structure and a hardness of HV 5 at 550 to 920 Kg/mm.sup. 2.